The present embodiments relate to an electric amplifier and method for the control thereof.
An electric amplifier is often constructed using two stages. In a first stage, an intermediate circuit generator or a power pack generates a supply voltage of average precision. The power pack supplies an end stage, which generates an output signal with the desired property. The output signal, depending on various requirements, may have a voltage that is converted upward compared to the supply voltage. The output signal, depending on the use, may have predeterminable signal courses that are either constant or that vary over time.
An electric amplifier may be used as a gradient amplifier for gradient coils, for example, in magnetic resonance systems. For example, quickly changing courses of output signals over time should be adhered to precisely. Switch elements, which are provided in the end stage of a bridge circuit, may be triggered in such a way that when the required supply voltage is supplied, the output signal with the desired properties can be generated.
The precision with which the desired parameter values of the output signal can be adhered to depends on the properties of a trigger device that triggers the end stage switch elements of the electric amplifier.
The trigger device includes a control unit that triggers the switch elements, and a regulator. The switch elements may be, for example, semiconductor components. The regulator, which is connected upstream of the control unit, makes a closed-loop control signal available to the control unit as a function of the output signal output by the end stage. The regulator, the control unit, and the end stage forma closed-loop control circuit. The regulator compares a desired-value parameter with a regulating or actual-value variable measured using the output signal of the end stage. The regulator, as a function of a standard deviation, outputs a controlled-value variable. Standard deviation is understood to be the difference between the desired-value parameter and the actual-value variable. When the actual-value variable is increasing, the standard deviation assumes a negative value. The regulator reduces the controlled-value variable to an increased extent. The decrease in the controlled-value variable counteracts the increase in the actual-value variable. This is known as negative feedback.
If an incorrect setting of the regulator causes the negative feedback to change into a positive feedback, then the closed-loop control circuit of a regulator amplification, for example, becomes unstable and begins to oscillate. The term positive feedback is when the controlled-value variable, which corresponds to the closed-loop control signal output by the regulator, also rises with an increasing actual-value variable. A positive feedback may be caused by the regulator. The positive feedback may be caused by, for example, overly high regulator amplification and is expressed in a constantly rising amplitude and frequency of the closed-loop control signal, which causes the regulator to overamplify. Because of the closed-loop control signal that oscillates in positive feedback of the regulator, components in the control unit may become damaged or overloaded because of the wide amplitude at the elevated frequency.